1. Field of the Invention
This invention relates to a power switchover apparatus for switching over the power source from a motor to an engine in a hybrid vehicle of the parallel type or the parallel and serial combined type wherein a drive shaft of the engine and a drive shaft of the motor are mechanically connected to each other.
2. Description of Background Art
An electric automobile using a motor as a power source provides no atmospheric pollution and low noise pollution and is high in acceleration and deceleration when compared with a conventional automobile, which uses an engine as a power source. However, presently, the cruising distance of an electric automobile is sometimes short due to a limitation in capacity of the batteries. Therefore, a hybrid vehicle, which uses a motor and an engine, has been put into practical use.
As such hybrid vehicles, those of a “series hybrid type”, a “parallel hybrid type”, and a “series-parallel combined type” are generally known. The hybrid vehicle of the series hybrid type uses a motor as the power source and uses an engine as a power source for a generator for charging the battery. The hybrid vehicle of the parallel hybrid type uses both a motor and an engine as the power source for the vehicle and uses them in response to a traveling condition and so forth. The hybrid vehicle of the series-parallel combined type selectively uses the two types in response to a traveling situation.
Since a motor is superior in low speed torque and efficiency in a low speed region when compared to an engine, a hybrid vehicle of the parallel hybrid type or the series-parallel combined type performs switchover control of the power source such that the motor is used as the power source in a low speed region whereas the engine is used as the power source in a middle or high speed region.
However, if the torque of the engine is added simply to the torque of the motor upon such power switchover, then the driving torque of the vehicle changes suddenly and a switchover shock occurs. Also when the rotational speeds of the engine and the motor do not coincide with each other, the driving torque of the vehicle changes suddenly and a switchover shock occurs similarly.
In order to reduce such a shock upon power switchover as just described, Japanese Patent Laid-Open No. 2000-23311 discloses a technique wherein an electromagnetic power clutch is interposed between an engine and a drive motor. When the power source is to be switched over, the output power of the motor is controlled so that the rotational speeds and the output torques of the engine and the drive motor may coincide witch each other. Then, at a point of time when the rotational speeds and the output torques become coincident with each other, exciting current is supplied to the clutch to establish connection of the engine and the drive motor.
According to the background art described above, it is necessary to provide an electromagnetic clutch between the engine and the drive motor, and it is necessary to provide a feed line for supplying exciting current to the electromagnetic clutch and a control apparatus for controlling the excitation current. Therefore, the background art described above has a configuration that is complicated and a weight that is increased.